Picocell and Femtocell deployment is an important feature in the next generation (4G) wireless mobile communication systems, such as WiMAX 2.0 systems defined by IEEE 802.16m and LTE-Advanced systems defined by 3GPP Release 10. The deployment of pico/femtocell enhances indoor services of wireless mobile communication, off-loads traffic of macro and/or micro base stations, and compensates and reduces outdoor service coverage holes. Network timing synchronization becomes an important issue when pico/femtocells are deployed together with overlaying macro/microcells, especially in a co-channel development scenario. Network timing synchronization has to be kept so that radio signals from a pico/femto base station and an overlaying macro/micro base station over the air do not interfere with each other.
FIG. 1 (Prior Art) illustrates a hierarchical cell structure of macro/microcells and pico/femtocells in a cellular OFDM communication system 10. Cellular OFDM communication system 10 comprises a macro/micro base station BS 11, pico/femto base stations BS 12 and BS 13, and mobile stations MS 14 and MS 15. The two pico/femto base stations BS12 and BS13 have smaller cell coverage, while the overlaying macro/micro base station BS11 has much larger cell coverage. In the example of FIG. 1, BS12 is located close to BS11 while BS13 is located far from BS11. Mobile station MS14 is served by pico/femto BS13, and mobile station MS15 is served by macro/micro BS11. MS15 is located far from its serving BS11 but relatively close to pico/femto BS13.
FIG. 2 (Prior Art) illustrates downlink and uplink subframes and transmission timing in cellular OFDM communication system 10. As illustrated in FIG. 2, in a time division duplex (TDD) system, macro base station BS11 transmits data during downlink (DL) subframes and receives data during uplink (UL) subframes. Each DL subframe is followed by an UL subframe after a predefined transmit transition gap (TTG) time, and each UL subframe is followed by a DL subframe after a predefined receive transition gap (RTG) time. Pico/femto base station BS12 is synchronized with overlaying BS11, and has approximately the same DL and UL transmission timing due to its physical proximity with BS11. Pico/femto base station BS13 is also synchronized with overlaying BS11, but has a delayed DL and UL transmission timing due to DL propagation delay. For mobile station MS15 that is served by BS11, it receives data during DL subframes and transmits data during UL subframes. After DL and UL synchronization with its serving BS11, each DL subframe of MS15 synchronizes with each DL subframe of BS11 with a DL propagation delay, while each UL subframe of MS15 synchronizes with each UL subframe of BS11 with an UL timing advance. Because of the DL propagation delay and the UL timing advance, it can be seen that the UL subframes between pico/femto BS13 and MS15 has a timing difference, as illustrated in FIG. 2.
Therefore, it is necessary for pico/femto BS13 to adjust its TTG to avoid possible uplink interference between a pico/femto mobile station (e.g. MS14) and a nearby macro/micro mobile station (e.g. MS15). Without adjustment of TTG in a pico/femtocell, the uplink receiving of the pico/femto base station may be interfered by uplink transmission of neighboring mobile stations served by an overlaying macro/micro base station due to non-aligned uplink subframes if the timing difference is not an integer number of OFDM symbols. For mobile stations that handover or are camped on in the pico/femtocell, however, they have no knowledge of any TTG adjustment. Without knowing the TTG adjustment value, there will be different understanding about the UL transmission time between mobile stations and the pico/femto BS, which would likely to result in incomplete receiving of uplink signals such as ranging signals at the pico/femto BS.
In current IEEE 802.16m and 3GPP systems, non-synchronized ranging channel (RCH) or random access channel (RACH) is utilized for initial and handover ranging in pico/femtocells. FIG. 3 (Prior Art) illustrates a non-synchronized ranging channel 31 and a data channel 32 in an IEEE 802.16m OFDM system. As illustrated in FIG. 3, a pico/femto MS utilizes non-synchronized ranging channel 31 for ranging preamble transmission, and a macro/micro MS utilizes data channel 32 for data transmission. The non-synchronized ranging channel 31 has a long ranging cyclic prefix (RCP) length and guard time to accommodate the time difference of non-aligned uplink subframes between the pico/femto MS and the macro/micro MS. This solution, however, is associated with a few disadvantages. First, it requires a non-synchronous ranging channel to have a different CP length from a data channel in the same communication system. Second, the different CP lengths between non-synchronous ranging channel and data channel may result in interference with each other. As illustrated in FIG. 3, since the time domain structure of the non-synchronous ranging channel 31 is different from the data channel 32, the orthogonality in frequency domain between the two channels may be destroyed. When the macro/micro MS transmits data channel, and the pico/femto MS transmits ranging channel, ranging performance in pico/femtocell degrades significantly. Third, a non-synchronous ranging channel may have a different physical structure and code sequences than those of a synchronous ranging channel. Thus, without utilizing a unified synchronous ranging channel, hardware complexity and cost of a pico/femto BS may not be reduced.